The specific conductance or electrical conductivity (SC, μS/cm) of a solution
is easy to measure and very useful for
checking parameters of the aqueous model or defining association
constants for new species.
PHREEQC calculates the SC of a solution from
the concentration and the diffusion coefficient of the charged species, and applies a correction for ionic strength. Results obtained with the traditional ionassociation and the Pitzer model agree very well with measurements of a large variety of solutions, spanning a range of conductances from 10 to 160 000 μS/cm.
The figure compares data provided by P.J. Stuyfzand with calculated values.
The principle of the calculation is that the molar conductivity of a solute species and its diffusion coefficient are related by:
PHREEQC's calculations were checked with data from the Handbook of Chemistry and Physics, which lists
SC's for salt solutions at 20^{o}C.
PHREEQC prints SC in the output file and the special BASIC variable SC provides it in keywords USER_PUNCH or _GRAPH. Results calculated with PHREEQC input file salt_sc.phr are graphed on the right. Note that the calculations need diffusion coefficients given under SOLUTION_SPECIES in PHREEQC.DAT and PITZER.DAT. The diffusion coefficients are corrected to temperature T (Kelvin) of the solution with: (D_{w})_{T} = (D_{w})_{298} × exp(d / T  d / 298) × (T / 298) × (η_{298} / η_{T}), where d is a coefficient and η is the viscosity of water. The coefficients d, a_{1} and a_{2} are entered with parameter dw in SOLUTION_SPECIES. 
